Inductive coupling is often used to wirelessly transfer power between two coils when a wired connection is not convenient or practical. Inductive coupling is made possible by four primary components: a source, a generating coil, a receiving coil, and a drain. The source provides a current which flows through the generating coil to produce an electromagnetic field. The electromagnetic field propagates from the generating coil to the receiving coil and induces a voltage across the receiving coil which is then used to power the drain.
Inductive coupling eliminates the need for direct connections to transfer power and allows for both coils to be fully enclosed and protected. In addition to the convenience of removing external wires, this approach is also used in situations where conditions are prohibitive to conventional electrical connections, such as when the device needs to be impervious to its environment. Devices such as electric toothbrushes, pacemakers, and cochlear implants employ inductive coupling to receive energy wirelessly.
Since the efficiency of the system is primarily dependent upon the strength of the electromagnetic field at the receiving coil, the system is sensitive to misalignment of the coils and further degradation of the field due to uncontrolled propagation. In all the listed applications, the generating coil is placed in close proximity to the receiving coil to charge the device because the effective operable area of energy transfer is relatively small.
Ferrite backings have previously been placed on coils to improve the efficiency of power transfer between coils by reducing the leakage of the electromagnetic field. However, these ferrites are expensive, heavy, and brittle. Furthermore, such systems do not perform well outside of ideal situations and still suffer significant efficiency deterioration when the coils are even slightly misaligned.
Inductive coupling becomes dramatically less efficient if the coils are misaligned or as the distance between the coils increases. Due to these shortcomings, inductive coupling has seen limited use as an alternative to direct wired connections for power transfer.